1. Field of the Invention
The present invention relates to a predistortion type distortion compensation apparatus for compensating the distortion occurring in an amplifier for amplifying an input signal by the control using a control value corresponding to the level of the input signal, and more particularly to a technique of updating the control value to a more precise value.
2. Description of the Related Art
For example, in a base station apparatus (CDMA base station apparatus) of a mobile communication system conforming to a W-CDMA (wide-band code division multiple access) mobile communication method, the radio signal must be sent to a mobile station apparatus (CDMA mobile station apparatus) physically at a long distance, and the transmission signal must be sent out by largely amplifying by an amplifier.
However, the amplifier is an analog device, and there is limitation in amplification. This limitation in amplification is also known as a saturation point, and after the saturation point, the output power is unchanged if the power entered in the amplifier is increased, and the output is nonlinear. This nonlinear output causes a nonlinear distortion.
FIG. 7 shows an example of spectrum of transmission signal before input into the amplifier, and FIG. 8 shows an example of spectrum of the transmission signal amplified and issued by the amplifier when distortion is not compensated. In FIG. 7 and FIG. 8, the axis of abscissas of the graph indicates the frequency (in kHz) and the axis of ordinates denotes the power ratio (in dB).
As shown in FIG. 7, in the transmission signal before amplification, the signal component out of desired signal band is suppressed to a low level by the band limiting filter, but in the signal after passing through the amplifier, as shown in FIG. 8, the signal is distorted, and signal component has leaked to outside of the desired signal band (adjacent channel).
For example, in the base station apparatus, since the transmission power is high as mentioned above, the magnitude of leak power to adjacent channel is strictly regulated, and it is an important problem how to curtail such adjacent channel leak power (ACP).
As means for curtailing the adjacent channel leak power, an example of a transmission power amplifier with distortion compensation used in a conventional base station apparatus is explained.
FIG. 9 is a structural example of such transmission power amplifier with distortion compensation, and its operation is explained.
In the transmission power amplifier with distortion compensation, a transmission signal (including I component and Q component) generated in a base band signal generator 61 is put into a vector adjusting unit (predistortion unit) 62 and a power measuring unit 69, and the transmission signal entering the vector adjusting unit 62 is compensated of distortion by this vector adjusting unit 62. The vector adjusting unit 62 is generally composed of a complex multiplier, and is designed to compensate the distortion of the transmission signal, according to the control from a controller 68 described below, by setting the amplitude-phase plane characteristic so as to be a reverse characteristic of the nonlinear characteristic of an amplifier 64 described below, and giving this characteristic (that is, the reverse characteristic) to the transmission signal as the distortion compensation characteristic.
The transmission signal compensated of distortion by the vector adjusting unit 62 is up-converted from the base band to the carrier frequency band by a transmission modulator 63, and is amplified by the amplifier 64, and is supplied into the antenna not shown.
In the amplifier 64, distortion occurs when amplifying the transmission signal, and the amplifying apparatus with distortion compensation is provided with a feedback system for detecting the residue of distortion for observing whether the distortion is compensated properly or not.
This feedback system comprises a local frequency generator 65, a demodulator 66, and an A/D converter 67, and part of the output signal (amplified signal) of the amplifier 64 supplied into the antenna is, for example, taken out by a directional coupler, and put into the demodulator 66.
In the feedback system, the amplified signal entered from the directional coupler into the demodulator 66 is demodulated by using a local signal entered from the local frequency generator 65 into the demodulator 66, and the demodulated signal is converted from analog signal into digital signal by the A/D converter 67, and the digital signal is put into the controller 68.
In the power measuring unit 69, the power of transmission signal (transmission power) entered from the base band signal generator 61 is detected, and the detection result is sent to the controller 68.
The controller 68 is composed of, for example, DSP (digital signal processor), and it detects the residual distortion amount from the digital signal entered from the A/D converter 67, and controls the vector adjusting unit 62 so that the distortion may be compensated properly by the vector adjusting unit 62 on the basis of the detection result. In this control, it is controlled so that the distortion compensation characteristic corresponding to the transmission power noticed from the power measuring unit 69 may be used in the distortion compensation.
As described herein, according to the transmission power amplifier with distortion compensation shown in FIG. 9, appropriate distortion compensation is done for the distortion occurring in the amplifier 64, and an efficient transmission power amplification process is realized.
FIG. 10 shows an example of spectrum of the transmission signal amplified and issued by the amplifier 64 when distortion is thus compensated, and in this signal spectrum, the adjacent channel leak power is curtailed substantially. The axis of abscissas of the graph indicates the frequency (in kHz) and the axis of ordinates denotes the power ratio (in dB).
Several prior arts relating to such distortion compensation are presented below.
For example, in the digital wireless apparatus disclosed in Japanese Laid-open Patent No. 9-294144, distortion is compensated by using a feedback system similar to the one shown in FIG. 9, and in this feedback system, same as in FIG. 9, the required signal to be transmitted (the original transmission signal) is down-converted together with undesired signal generated in adjacent channel (that is, the distortion generated in the amplifier), and all signals are processed by orthogonal demodulation or the like.
In the automatic follower type predistorter disclosed in Japanese Patent Publication No. 63-10613, distortion occurring in the amplifier is compensated by using the feedback system similar to the one shown in FIG. 9, and this feedback system, similarly, demodulates the amplified signal including the transmission signal band (that is, reproduces the base band signal before modulation), and processes by A/D conversion or the like.
Although not intended to compensate distortion, in the measuring apparatus and measuring method of adjacent channel leak power disclosed in Japanese Laid-open Patent No. 9-138251, same as above, the carrier signal (corresponding to the required signal) is taken out together with the adjacent channel signal (corresponding to undesired signal), and they are processed by fast Fourier transform (FFT), and the adjacent channel leak power ratio (power ratio of required signal and undesired signal).
As in distortion compensation of the transmission power amplifier with distortion compensation shown in FIG. 9, the predistortion system is known as one of the methods of compensating distortion occurring in the amplifier. In the predistortion system, the reverse characteristic of the nonlinear characteristic of the amplifier is preliminary given to the signal to be entered in the amplifier, and distortion occurring in the amplifier is compensated. The nonlinear characteristic of the amplifier includes the AM-AM characteristic changing the level of output signal nonlinearly depending on the level of input signal, and AM-PM characteristic changing the phase of output signal nonlinearly depending on the level of input signal.
FIG. 11 shows other circuit example of the apparatus employing such predistortion system (predistortion type distortion compensation amplifying apparatus), and this predistortion type distortion compensation amplifying apparatus comprises a predistortion unit 71 including a variable attenuator (ATT) 81 and a variable phase shifter 82, a power amplifier (PA) 72 composed of one or plural power amplifiers, an envelope detector 73, a compensation table 74 including a table for amplitude control (ATT table) 83 corresponding to the variable attenuator 81 and a table for phase shift control (phase shifter table) 84 corresponding to the variable phase shifter 82, a side band power measuring unit 75, a specific time integrating circuit 76, and a control circuit 77.
An example of operation of the shown predistortion type distortion compensation amplifying apparatus is shown.
The signal entering from the input end of the predistortion type distortion compensation amplifying apparatus is distributed into two signals, and one distribution signal is put into the variable attenuator 81 of the predistortion unit 71, and other distribution signal is put into the envelope detector 73.
The envelope detector 73 detects the envelope information of the input signal (momentary power level), and issues the result of detection to the compensation table 74.
The compensation table 74, referring to the ATT table 83, reads out the control value for amplitude control corresponding to the envelope information entered from the envelope detector 73, and issues the control value to the control terminal of the variable attenuator 81 of the predistortion unit 71 as a control signal for amplitude compensation.
Further, the compensation table 74, referring to the phase shifter table 84, reads out the control value for phase shift control corresponding to the envelope information entered from the envelope detector 73, and issues the control value to the control terminal of the variable phase shifter 82 of the predistortion unit 71 as a control signal for phase shift compensation.
The ATT table 83 and phase shifter table 84 are composed of memories for storing, for example, the envelope information as reference address, and control value corresponding to the reference address. The compensation table 74 reads out the control value corresponding to the address, the address being the envelope information entered from the envelope detector 73, from the ATT table 83 and phase shifter table 84, and issues to the variable attenuator 81 and variable phase shifter 82 of the predistortion unit 71.
The variable attenuator 81 in the predistortion unit 71 attenuates the amplitude of the input signal by the attenuation amount controlled by the control signal entered from the compensation table 74, and issues to the variable phase shifter 82.
The variable phase shifter 82 in the predistortion unit 71 changes (shifts) the phase of the signal entered from the variable attenuator 81 by the phase shift amount controlled by the control signal entered from the compensation table 74, and issues to the power amplifier 72.
Thus, in the predistortion unit 71, the input signal is corrected (compensated) of amplitude or corrected (compensated) of phase depending on the envelope information of the input signal, and the input signal after correction is issued to the power amplifier 72.
The power amplifier 72 amplifies the input signal after predistortion entered from the variable phase shifter 82 of the predistortion unit 71, and issues the amplified signal from the output end of the predistortion type distortion compensation amplifying apparatus.
The side band power measuring unit 75, specific time integrating circuit 76, and control circuit 77 process to optimize the control value to be stored in the ATT table 83 and phase shifter table 84 of the compensation table 74.
Specifically, the side band power measuring unit 75 couples and receives a part of the amplified signal issued from the power amplifier 72 from, for example, the directional coupler, measures the adjacent channel leak power (side band component power) included in the received amplified signal, and issues the measured power to the specific time integrating circuit 76. The component of the adjacent channel leak power includes the distortion component generated in the power amplifier 72.
The specific time integrating circuit 76 integrates the adjacent channel leak power entered from the side band power measuring unit 75 for a predetermined specific time, and issues the integration result to the control circuit 77. Herein, the integration result of the adjacent channel leak power is issued to the control circuit 77, but it may be also designed, for example, to average the adjacent channel leak power issued from the side band power measuring unit 75 for a predetermined specific time, and issue the averaged result to the control circuit 77, and in this case the averaged result corresponds to the average of the integration result in the specific time.
The control circuit 77 updates the control values stored in the ATT table 83 and phase shifter table 84 of the compensation table 74, for example, by the following updating method on the basis of the integration result of the adjacent channel integration result entered from the specific time integrating circuit 76.
The ATT table 83 compares the integration result obtained from the specific time integrating circuit 76 when, for example, part of the control value stored in the ATT table 83 is changed and the distortion is compensated by using the changed control value of the ATT table 83, and the integration result obtained from the specific time integrating circuit 76 when the distortion is compensated by using the control value of the ATT table 83 before the change. The control value of the ATT table 83 in the case of obtaining the smaller integration result of the two integration results compared is used as a more appropriate table value. By repeating such operation, the control value stored in the ATT table 83 is brought closer to an optimum value.
In the phase shifter table 84, by similar operation, the control value stored in the phase shifter table 84 is brought gradually to an optimum value.
Preferably, the control values stored in the ATT table 83 and phase shifter table 84 should be updated so that the integration result obtained in the specific time integrating circuit 76 may be minimum.
Referring now to FIGS. 12(a)-12(e), the principle of compensating the distortion occurring in the amplifier is explained.
FIG. 12(a) shows an example of gain characteristic P1 corresponding to the input signal level (input level) and an example of characteristic Q1 of phase change corresponding to the input level, as the characteristic of the amplifier, in which the axis of abscissas denotes the input level, and the axis of ordinates represents the gain and phase changes. As shown in FIG. 12(a), the gain and phase changes of the amplifier are constant in the linear region of a relatively small input level, but the changes are nonlinear depending on the input level in the nonlinear region where the input level is relatively large.
FIG. 12(b) shows, as the input and output characteristic of the amplifier, an example of characteristic P2 of level (output level) of the output signal corresponding to the input level, and an example of characteristic Q2 of phase (output phase) of the output signal corresponding to the input level. As shown in FIG. 12(b), in the above linear region, the input level and output level are proportional, and the output phase is constant, but in the nonlinear region, the output level and output phase vary nonlinearly depending on the input level. In such amplifier, an amplitude distortion is generated by the nonlinear change of the output level, and a phase distortion is generated by the nonlinear change of the output phase.
On the other hand, FIG. 12(c) shows an example of characteristic P3 of compensation gain corresponding to the input level and an example of characteristic Q3 of compensation phase change corresponding to the input level, as the compensation characteristic for compensating the amplitude distortion and phase distortion occurring in such amplifier, in which the axis of abscissas denotes the input level and the axis of ordinates represents the compensation gain and compensation phase changes. The characteristic P3 of compensation gain and characteristic Q3 of compensation phase change shown in FIG. 12(c) have the characteristic (reverse characteristic) for canceling the gain P1 and phase change Q1 of the amplifier shown in FIG. 12(a), and by applying the compensation gain P3 and compensation phase change Q3 having such reverse characteristic to the signal to be amplified by the amplifier, generally, the amplitude distortion and phase distortion generated in the amplifier can be compensated.
FIG. 12(d) shows an example of characteristic P4 of gain to input level and an example of characteristic Q4 of phase change to input level, as the characteristic in the case of such compensation of amplitude distortion and phase distortion occurring in the amplifier, in which the axis of abscissas denotes the input level and the axis of ordinates represents the gain and compensation phase changes. As shown in FIG. 12 (d), in the characteristic after distortion compensation, if the input level is relatively large, the gain and phase changes are constant.
FIG. 12(e) shows an example of characteristic P5 of output level to input level and an example of characteristic Q5 of output phase to input level, as the input and output characteristic in the case of compensation of amplitude distortion and phase distortion occurring in the amplifier, in which the axis of abscissas denotes the input level and the axis of ordinates represents the output level and output phase. As shown in FIG. 12(e), in the input and output characteristic after distortion compensation, if the input level is relatively large, the output level is proportional to the input level, and the output phase remains invariable.
In this way, to the signal amplified by an amplifier having characteristic g, by giving compensation characteristic f of reverse characteristic of the characteristic g, the distortion of this signal can be compensated.
In the ATT table 83 and phase shifter table 84 of the compensation table 74, control values for realizing such compensation characteristic f are stored.
An example of prior art relating to predistortion is given below.
For example, in the predistortion of the amplifier and the amplifying device disclosed in Japanese Laid-open Patent No. 2000-78037, as a manner of predistorting by preliminarily deforming the input signal of the amplifier, the input signal is deformed by the compensation coefficient corresponding to differential or integral or both values of the input signal, and the adjacent channel leak power is reduced by broadening the signal band, and the frequency characteristic in the base band is improved.
In the predistortion device and its method disclosed in Japanese Laid-open Patent No. 2000-201099, by predistorting by using a predistortion function obtained from the inverse function of the input and output characteristic holding the inclination of small power portion of favorable linearity to the input and output characteristic of the power amplifier, a precise distortion is realized. Specifically, an arbitrary straight line (for example, function y=x) is set as the reference, the predistortion unit is roughly controlled by using table values of two stages and then controlled finely, and the error from the reference is minimized. This publication includes an embodiment in the base band and an embodiment in radio frequency (RF) band.
However, in the predistortion type distortion compensation amplifying apparatus as shown in FIG. 11, since the control value of the compensation table 74 is updated on the basis of the integration result of the adjacent channel leak power observed for a specific time regardless of the level of input signal, when processing a signal, such as CDMA signal, which is large in peak factor of input signal and low in frequency of occurrence of specific input level, the ratio of components corresponding to the input level of such low frequency of occurrence contained in the integration result of adjacent channel leak power is small.
Accordingly, when updating the control value corresponding to the input level of low frequency of occurrence, the density of the adjacent channel leak power component corresponding to such input level is low in the integration result, and the control value of the compensation table 74 cannot be updated to a proper value, or it takes much in convergence into optimum control value, or the control value does not converge.
The invention is devised to solve such problems of the prior arts, and it is hence an object thereof to present a predistortion type distortion compensation apparatus capable of updating the control value to a precise value, when compensating the distortion occurring in an amplifier for amplifying an input signal by controlling by using a control value corresponding to the level of the input signal.